Semiconductor devices and liquid crystal display devices are produced by means of the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. In view of the above, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to form a liquid immersion area so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
In order to satisfactorily perform the liquid immersion exposure process and various types of optical measuring processes through the liquid, it is important that the liquid immersion area of the liquid is formed in a desired state. For example, there is such a possibility that the substrate and/or the substrate stage may be slightly deformed due to the fluctuation of the pressure of the liquid in the liquid immersion area, and the exposure accuracy and the measurement accuracy may be deteriorated by the deformation. In another situation, when the pressure fluctuation of the liquid arises, a part of the projection optical system (for example, the optical element disposed on the side most closely to the image plane), which makes contact with the liquid, may be displaced and/or vibrated. As a result, the image of the pattern to be projected onto the substrate is deteriorated, and/or the measurement accuracy is deteriorated via the projection optical system and the liquid.
Further, when the liquid is supplied in order to form the liquid immersion area, there is such a high possibility that any gas portion such as the bubble may be formed in the liquid of the liquid immersion area. If the gas portion is formed in the liquid of the liquid immersion area, for example, the following phenomenon arises. That is, the exposure light beam, which is radiated to form the image of the pattern on the substrate, does not arrive at the surface of the substrate. In another situation, the exposure light beam, which is radiated to form the image of the pattern on the substrate, does not arrive at a desired position on the substrate. In still another situation, the measuring light beam does not arrive at the measuring unit. In still another situation, the measuring light beam does not arrive at a desired position. As a result, the exposure accuracy and the measurement accuracy are deteriorated.
On the other hand, when the liquid immersion area of the liquid is formed on the substrate by supplying and recovering the liquid by using the liquid supply mechanism and the liquid recovery mechanism, there is such a possibility that the following inconvenience may arise as well. That is, the liquid immersion area is not formed in a desired state due to the occurrence of any abnormality in the exposure apparatus, for example, such that the liquid supply mechanism and/or the liquid recovery mechanism malfunctions. For example, if the liquid immersion area is increased to be larger than a predetermined size, there is such a high possibility that the liquid may outflow to the outside of the substrate. Further, there is such a possibility that the following situation may arise. That is, the liquid cannot be retained satisfactorily on the image plane side of the projection optical system depending on the condition of the movement of the substrate stage. This also causes the inconvenience such that the gas portion is formed in the liquid immersion area, and/or the liquid outflows to the outside of the substrate. If the liquid outflows, the following inconvenience arises due to the outflow liquid as well. That is, for example, any mechanical part, which is disposed around the substrate stage for supporting the substrate, is rusted, and/or the electric leakage occurs, for example, in the stage-driving system. Further, if the liquid outflows, for example, the environment (temperature, humidity), in which the substrate is placed, is varied due to the vaporization of the outflow liquid. As a result, the substrate and/or the substrate stage is thermally deformed. In another situation, the vaporization of the liquid causes any fluctuation in the gas (air) on the optical paths for the various types of measuring light beams for measuring, for example, the position information about the substrate. As a result, the exposure accuracy and/or the measurement accuracy is deteriorated. Further, if any natural disaster such as the thunder and the earthquake and/or any unexpected accident arises, the outflow of the liquid occurs as described above due to the malfunction of the liquid recovery unit caused by the power failure of the power source of the exposure apparatus.